Butyl rubber (IIR), a random copolymer of isobutylene and isoprene is well known for its excellent thermal stability, ozone resistance and desirable dampening characteristics. IIR is prepared commercially in a slurry process using methyl chloride as a vehicle and a Friedel-Crafts catalyst as the polymerization initiator. The methyl chloride offers the advantage that AlCl3, a relatively inexpensive Friedel-Crafts catalyst, is soluble in it, as are the isobutylene and isoprene comonomers. Additionally, the butyl rubber polymer is insoluble in the methyl chloride and precipitates out of solution as fine particles. The polymerization is generally carried out at temperatures of about −90° C. to −100° C. See U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993, pages 288-295. The low polymerization temperatures are required in order to achieve molecular weights which are sufficiently high for rubber applications.
Typically, commercial grades of IIR possess unsaturation levels of approximately 2 mol %. While this degree of unsaturation is commensurate with material stability, it also limits the cure reactivity of these polymers. The post-polymerization halogenation of the isoprene units found in IIR with either elemental Chlorine or Bromine results in the isolation of either Chlorobutyl (CIIR) or Bromobutyl (BIIR). These materials possess extremely reactive allylic halide sites which significantly enhance their rate of cure. Co-pending Canadian Patent Applications CA 2,386,098, CA 2,383,474, CA 2,368,363, CA 2,418,822, CA 2,465,301 and CA 2,471,006 disclose the ability to exploit the allylic bromide functionality present in BIIR in amine- and phosphine-based nucleophilic substitution reactions. The resulting substituted BIIR possesses enhanced levels of interaction with siliceous fillers and can be successfully incorporated in silica reinforced formulations.
While nucleophilic substitution occurs quite readily with neutral amines and phosphines, the analogous reactions with oxygen or sulfur based nucleophiles are much more arduous. The use of oxygen and sulfur based nucleophiles often requires the presence of a strong base, such as an alkali metal hydroxide, to yield the corresponding anionic nucleophile. Even though the depronated oxygen (or sulfur) nucleophile possesses the required level of nucleophilicity, its ionic nature limits its solubility in apolar polymer matrices such as BIIR. Consequently, solvents of intermediate polarity (e.g. THF, dichloromethane) are often used to facilitate such reactions.
From both an environmental and industrial perspective, it would be desirable to carry out such nucleophilic substitution reactions under solvent-free conditions. However, it is first necessary to overcome the solubility limits associated with the use of most deprotonated oxygen or sulfur nucleophiles. Phase transfer catalysis can offer a solution in overcoming the solubility limits.
Typically, phase transfer catalysis involves the introduction of catalytic amounts of a phase transfer catalyst (PTC), such as tetrabutylammonium bromide or trioctylmethylammonium chloride, (Aliquot® 336) into a solution containing an alkali metal salt of a nucleophile and the reactive substrate. Exchange of the alkali metal cation for either a tetrabutylammonium or trioctylmethylammonium counter-ion increases the solubility of the nucleophile in the dissolved rubber phase and ultimately increases the efficiency of the nucleophilic substitution reaction. See for example, Dehmlow, E. V.; Dehmlow, S. S. Monographs in Modern Chemistry No 11: Phase Transfer Catalysis, 2nd ed.; Verlag Chimie: Germany, 1983. Fréchet, J. M. J.; de Smet, M. D.; Farrall, M. J. J. Org. Chem. 1979, 44, 1774-1779; b) Fréchet, J. M. J. J. Macromol. Sci.-Chem. 1981, A15, 877-890. Nishikubo, T.; Iizawa, T.; Kobayashi, K.; Masuda, Y.; Okawara, M. Macromolecules 1983, 16, 722-727.